The proposal will determine whether 60-Hz magnetic field (MF) exposures have biological effects that are detrimental to reproductive development. The experimental plan, in part, will define the MF exposure conditions that may influence a variety of endpoints that are relevant to the central nervous system control of sexual maturation. Cellular and molecular markers will be used to test a hypothesis about the mechanism by which MF exposures may influence reproductive development. In the Djungarian hamster, the process of puberty can be induced or arrested by environmental photoperiod. In long days, a critical period for initiation of puberty (15-25 days of age) is associated with rapid gonadal development, a dramatic elevation in gonadotropin secretion and increased numbers of morphologically unipolar GnRH neurons in the medial preoptic and diagonal band regions of the brain. Each of these indices are maintained a prepubertal levels by exposure to short days, an effect that is mediated by the circadian rhythm in pineal gland melatonin production. MF exposure, in a diverse number of species, including the Djungarian hamster, has been found to suppress the night-time rise in pineal melatonin production. If the effect of MF exposure on the melatonin rhythm is of physiological significance, then a disruption in the mechanism that conveys photoperiod information will result in an inappropriate response to short days by the reproductive system. Experiments are planned to focus on the mechanism of MF effects on melatonin and reproductive development. The objective is to determine whether MF exposure affects timekeeping by the endogenous biological clock that generates the melatonin rhythm or on the pineal itself. Studies of the molecular responses to MF exposure in the brain and pineal gland include transcriptional activation, as indicated by the expression of the immediate early genes, c-fos and NGFIA, as well as clock-related messages for arginine vasopressin homologues of the per gene. Further consideration of a direct cellular action of MF exposure on the GnRH neuron system at the onset of puberty and during its ontogeny leads to studies of GnRH cell number, distribution, morphology, migration, and projections during critical stages of development. The research approach uses an established animal model system to test hypotheses about the effects of MF exposure on biological timekeeping and the neural mechanism that controls reproductive development.